1. Field of the Invention
The present invention generally relates to a passenger detecting apparatus which is installed on a motor vehicle, automobile or the like for detecting situation or state of a passenger such as posture, body size, position, etc. to thereby output information concerning the detected state of the passenger, which information is then used for controlling operation of an air bag apparatus for protecting the passenger against injury upon occurrence of collision of the automobile, an air-conditioner employed for adjusting the temperature in the compartments, an alarm device designed for generating an alarm indicative of unsafety or insecurity situation such as dozing of the driver and the like. More particularly, the present invention is concerned with an improvement of the passenger detecting apparatus by implementing it in a thin structure. At this juncture, it should first be mentioned that the term “passenger” is used to conceptionally encompass not only the driver which is operating the automobile but also the person who is traveling in the automobile, i.e., the passenger(s) in the intrinsic sense.
2. Description of Related Art
In general, the automobile is equipped with a air bag apparatus including an air bag which is instantaneously inflated or expanded within a space intervening a steering wheel or dash-board and a passenger in order to protect him or her against injury upon occurrence of collision event. However, when the passenger is of a small size and operating the automobile at a position closer to the steering wheel or when a child is standing in front of a seat adjacent to the driver's (hereinafter this seat will also be referred to as the assistant's seat only for the convenience of description), i.e., when the distance between the steering wheel or the dash-board and the passenger is extremely short, expansion of the air bag will undesirably injure the passenger, far from protecting him or her, as is known in the art.
Besides, it is also known that in the case where the passenger occupying the assistant's seat is a child younger than six inclusive, expansion of the air bag should be suppressed from the standpoint of safety even when he or she is sitting in a normal posture. In this conjunction, it is further noted that in U.S.A., installation of the passenger detecting apparatus in the automobile in combination with the passenger protecting apparatus such as the air bag apparatus is planned to be statutorily enforced.
As the passenger detecting apparatus developed to this end, there may be mentioned the one disclosed in Japanese Patent Application Laid-Open Publication No. 309402/1997 (JP-A-9-309402). For having better understanding of the concept underlying the present invention, description will first be directed to this known passenger detecting apparatus. FIG. 5 of the accompanying drawings is a vertical sectional view of the passenger detecting apparatus described in the above-cited publication. Further, FIG. 6 is a sectional view taken along a line VI-VI in FIG. 5 as viewed in the direction indicated by arrows. In these figures, reference numeral 120 denotes a sensor unit which constitutes a major part of the passenger detecting apparatus and which is installed within a compartment of an automobile on a top wall at a position in the vicinity of an upper edge of a front window shield of the automobile in opposition to an assistant's seat.
Reference characters 21a, 21b and 21c denote light emitting elements, respectively. Reference numeral 28 denotes a supporting member for supporting the light emitting elements 21a, 21b and 21c on a supporting surface having a predetermined curvature so that optical axes of the light emitting elements 21a, 21b and 21c mutually intersect at a single point, as can be seen in FIG. 6. Reference characters 22a, 22b and 22c denote lenses which are provided in association with the light emitting elements 21a, 21b and 21c, respectively, for collimating light rays emitted from the light emitting elements 21a, 21b and 21c into parallel or collimated light beams 41a, 41b and 41c, respectively. Reference numeral 24 denotes a light receiving element. Further, reference characters 25a, 25b and 25c denote, respectively, condenser lenses for collecting or condensing on the light receiving element 24 the return light rays resulting from scattering of the light beams 41a, 41b and 41c upon striking on a passenger.
Next, operation of the conventional passenger detecting apparatus will be described. When the light beams 41a, 41b and 41c emitted from the light emitting elements 21a, 21b and 21c, respectively, are scattered upon striking on the passenger, parts of the scattered light rays are reflected backward to the sensor unit 120. More specifically, the scattered light rays will follow same optical paths as the light beams 41a, 41b and 41c as viewed orthogonally to the plane of FIG. 6 to impinge onto the lenses 25a, 25b and 25c to be focussed onto the light receiving element 24. On the other hand, viewing orthogonally to the plane of FIG. 5, the scattered light rays follow paths deviated by an angle α from those of the light beams 41a, 41b and 41c to impinge onto the lenses 25a, 25b and 25c, respectively, to be thereby focussed onto the light receiving element 24.
The angular difference a mentioned above will vary in dependence on the distance to the object at which the light beams are scattered. Accordingly, it is possible to determine the distance to the passenger in accordance with the principle of triangulation by employing as the light receiving element 24 such element as the PSD (Position Sensitive Detector) or the like which exhibits the optical position detecting function. Thus, on the basis of the position of the passenger detected by the passenger detecting apparatus, it can be decided that inflation or expansion of the air bag will incur insecurity far from protection when the position of the passenger is detected to be excessively close to the air bag. In that case, expansion of the air bag is suppressed even when the automobile should come into a head-on collision.
In the passenger detecting apparatus of the structure described above, it is generally required to employ the lens of relatively long focal distance in order to ensure high directional accuracy (i.e., precision of the directivity) of the light beam, which however gives rise to a problem that the length of the sensor unit 120 in the optical axis direction will then become increased. Particularly in the case where the passenger detecting apparatus is installed on the ceiling of the automobile, being orientated toward the passenger, the sensor unit 120 protrudes remarkably from the ceiling because of a large size of the sensor unit 120 in the heightwise direction, presenting a problem from the viewpoint of the inner space availability. Additionally, in the passenger detecting apparatus of the structure described above, there are required a number of lenses which corresponds to that of the light beams, which means that a relatively large number of lenses have to be used in order to ensure reliability for the operation of the passenger detecting apparatus. Needless to say, the cost of the passenger detecting apparatus will increase as the number of lenses increases, incurring high cost in manufacturing the apparatus.